FIG. 1A illustrates a conventional chip-to-chip signaling system 100 that employs transmit-side and receive-side equalization to compensate for channel imperfections. More specifically, transmitter 101 is implemented by a finite-impulse-response filter in which the bit to be transmitted and neighboring bits are loaded into a shift register 104 and multiplied by respective filter weights in multipliers 105. The multiplier outputs are summed (106) to produce a final signal that is driven onto a transmission-line signaling link 102 by line driver 107. By this arrangement, intersymbol interference and other channel effects may be compensated to produce a more open data eye (i.e., larger signal amplitude and/or duration) at the receiver 103. On the receive side, a decision-feedback equalizer (formed by amplifier 114, bit-slice circuit 115, shift register 116, filter-weight multipliers 117 and summing circuitry 118) contributes to the received waveform in a negative-feedback arrangement to further open the data eye, compensating for impedance discontinuities (e.g., at connectors 110a/110b or at the junction between the signaling link 102 and the transmitter 101 and/or receiver 103).
When operated in conjunction with a point-to-point signaling link or other signaling channel which is relatively free of reflection sources (e.g., stubs and impedance discontinuities), the equalized transmitter/receiver pair of FIG. 1A may achieve extremely high signaling rates, for example, approaching or exceeding 10 GHz. Unfortunately, as shown in FIG. 1B, the multi-drop signaling topologies common in memory systems and other high-bandwidth applications tend to exhibit band-limiting notches 121a-121z (i.e., intervals of attenuated frequency-response due at least in part to the reflection-inducing stubs at each drop along the signaling path), that often limit the top end signaling rate to the frequency of the lowest-frequency notch (e.g., ˜1 GHz as shown at 121a in the multi-drop frequency response of FIG. 1B).
FIG. 2 illustrates a recently proposed system 140 that employs multi-band signaling to exploit notch-bounded passbands in the signaling channel and thus overcome the notch-limited bandwidth of the baseband-only approach of FIG. 1A. More specifically, multiple data streams (X0-XN-1) are supplied to distinct transmission branches of transmitter 141, each transmission branch including a low-pass-filter 1430-143N-1 and (except for a baseband branch) up-converter 1441-144N-1 to generate spectrally-differentiated signals that may be wire-summed (145) and conveyed in respective notch-bounded passbands of the signaling channel. In the receiver 151, counterpart down-conversion (1531-153N-1) and low-pass filtering (1550-155N-1) operations are performed to recover multiple baseband signals which are supplied to respective bit-slice circuits 1570-157N-1 (i.e., circuits for distinguishing between signal levels) to recover the originally transmitted data streams.
While the multi-band signaling arrangement of FIG. 2 provides significant advantages over the base-band-only approach when faced with band-limiting notches, the multi-band transmitter 141 suffers an increased peak-to-average power ratio relative to the baseband-only transmitter of FIG. 1 and thus is less efficient in terms of energy-per-bit and therefore may less desirable in those instances in which a channel without band-limiting notches is available.